Trailer refrigeration units are controlled by a microprocessor which receives inputs indicating the temperature, humidity, etc. in the conditioned space and controls the refrigeration system responsive to the inputs. Additionally, the microprocessor records the inputs such that the history of the load during the trip is recorded. Accordingly, it is possible to determine when and why a load is spoiled, thawed or the like. Perishable cargo such as fruit, vegetables and flowers produce, and are affected by, gases. Ethylene, for example, is produced in the ripening of bananas and its presence promotes ripening while respiration of the “live” cargo produces carbon dioxide and requires oxygen. It is therefore preferable to introduce some fresh air with the recirculating air, where perishable cargo is present, if spoilage or premature ripening of the load is to be avoided.
In a trailer, the load normally occupies much of the available space such that the flow paths for the conditioned air are located in the space between the load and the trailer ceiling, walls, floor, and the channels defined by the pallets on which the load sits. To minimize the wastage of conditioned space, only the expansion device, the evaporator, the evaporator fan, necessary ducting and sensors are located in the conditioned space. The rest of the refrigeration unit and its controls are located on the exterior of the trailer and are powered by an external power supply such as a diesel engine or the unit may be connected to the electric grid.